HomeMy WebLinkAboutWPO201000017 Review Comments Stormwater Management Plan 2011-03-18ALg�,��
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COUNTY OF ALBEMARLE
Department of Community Development
401 McIntire Road
Charlottesville, Virginia 22902 -4596
Phone (434) 296 -5832 Fax (434) 972 -4126
Project:
North Pointe Northwest Residential Area
SDP - 2010 -00021 and WPO- 2010 -00017
Plan preparer:
Mr. Herb White, PE; W & W Associates
Owner or rep.:
Neighborhood Investments NP -LLC
Date received:
4 March 2010 (plan signed 3 March 20 10)
(Rev. 1) 27 July 2010
(Rev. 2) 3 February 2011
Date of Comment:
16 April 2010
(Rev. 1) 9 September 2010
(Rev. 2) 18 March 2011
Engineer:
Phil Custer
The second revisions to the final site, road, SWM, and ESC plans for the North Pointe Northwest
Residential Area project, received on 3 February 2011, have been reviewed. Engineering review can
recommend approval to the plan after the following comments have been addressed.
A. General review comments
1. This plan cannot be approved until the submittal for the road stream crossing is approved. The
ESC plan for this project will not be able to receive a grading permit until the crossing is
established.
(Rev. 1) Comment has been acknowledged by the applicant.
(Rev. 2) The technical aspects of the stream crossing ESC plan (WPO- 2009 - 00061) had been
approved in January of 2010 at the request of the applicant so an extension of the Army Corps
Permit could be received before an approaching deadline. That technical approval was not
given to all other aspects of the stream crossing plan: design of the culvert, FEMA approval,
mitigation approval, etc. All of these elements must be approved prior to the approval of this
site plan. [SP- 2006 -00034 Condition 2, SP- 2006 -00034 Condition 3, SP- 2006 -00034 Condition
5,17-3221
Per 17- 204.G, the approval of ESC plan for the stream crossing is now void. In the meeting
held between county staff and the applicant on June 2e 2010, it was discovered that I had
mistakenly allowed disturbance to the conservation area for the construction of a sediment trap
and it was agreed upon that sediment basin I would be constructed with the stream crossing
WPO plan, but no plan was ever submitted to correct the sequence. Please submit a new
application, fee, and a set of the revised stream crossing ESC plan showing the construction of
sediment basin 1.
2. The current ESC plan shows disturbance to critical slopes that were shown as being preserved in
the approved rezoning plan. Because of this, a critical slope waiver must be approved by the
Planning Commission before the current plan can proceed or the ESC plan must be redesigned to
stay within the limits shown on the plan approved by the Board of Supervisors.
(Rev. 1) The Director of Community Development has made the determination that a critical
slope waiver for the additional disturbance is not needed.
3. Property lines cannot go through buildings unless a substantial fire wall is constructed on the
boundary. A plat must be submitted and recorded prior to final site plan approval that rectifies this
problem. The simplest solution would be to combine the two parcels with a Boundary Line
Adjustment Plat.
(Rev. 1) It is my understanding that a Boundary Line Adjustment plat has been approved.
4. The plan appears to propose features within the 50ft construction easement granted to the owners
of TMP 32 -22K1 in the document recorded in DB 1663, PG 648. The applicant should remove
these features from the 50ft easement or amend the agreement so that it works for both parties.
(Rev. 1) All important features have been removed from this construction easement. Comment
has been addressed.
5. Please provide a note on the cover sheet of the plan which states that before a certificate of
occupancy is granted, a plat dedicating the ROW, public drainage easements with associated
deeds, stormwater management easements, and temporary construction easements must be
recorded.
(Rev. 1) Comment has been addressed.
6. Per Proffer 4.1, the County will request that the floodplain be dedicated once the Conditional
Letter of Map Revision is approved by FEMA and the county.
(Rev. 1) It is my understanding that the Conditional Letter of Map Revision is currently under
FEMA review. Please forward FEMA approval for this crossing to me once it is received. The
County Engineer has reviewed the CLOMR and will sign it as the county official when VDOT
approves the plan. This is being tracked with WPO- 2009 - 00061.
(Rev. 2) Comment remains unchanged. FEMA approval of the CLOMR will be needed prior to
the approval of the stream crossing plan. Approval of the stream crossing plan will be needed
prior to the approval of the site plan.
7. Per Proffer 4.4, please provide confirmation that all streambank mitigation required by the Army
Corps of Engineers for the Northwest Passage stream crossing is being provided onsite.
(Rev. 1) Comment has not been addressed. Please provide correspondence from the Army Corps
of Engineers stating that all mitigation can be provided on site.
(Rev. 2) The mitigation plan is being processed with the stream crossing application. The
mitigation plan must be approved prior to site plan approval. [17 -3221
B. Site Plan review comments (SDP- 2010 - 00021)
1. The placement of buildings 25, 26, and 27 is not in general accord with the approved rezoning
plan. Please work with the Planning Department to find a solution to this issue before the next
submittal.
(Rev. 1) The Planning Department is currently reviewing this variation request. It appears as
though this variation can be approved on the condition that SWM facility #10 is designed
satisfactorily.
(Rev. 2) Comment remains unchanged.
2. The distance between the private access easement to the church property and Discovery Court is
too short for VDOT to provide two entrance permits. These entrances must be either separated to
the minimum spacing required by VDOT or consolidated. The resolution to this issue should be
addressed simultaneously with the previous comment while in discussion with the Planning
Department to assure that the solution is still in general accord with the application plan.
(Rev. 1) It appears as though the entrance locations will meet the VDOT separation requirements.
County Engineering will defer to VDOT on this issue. In VDOT's last series of comments, it did
not appear this issue was raised. Please confirm the adequacy of the entrance locations with
VDOT.
(Rev. 2) Please refer to Comment #2 from Joel DeNunzio's most recent review (311812011).
The future entrance at the access easement's intersection with Lewis and Clark Drive will likely
not meet VDOT standards. I recommend that the applicant work with the northern property
owner to find a mutually beneficial accesslentrance arrangement for this corner of the site.
Please note that this issue may be raised with the latest Zoning Map Amendment plan.
3. The connection to TMP 32 -22KI must be design and constructed with this development. This
travelway must be no narrower than 20ft from curb to curb. The travelway must be design and
constructed along the existing access easement unless the access easement is modified. [18-
32.7.2.5]
(Rev. 1) The Director of Community Development has determined this connection does not
need to be constructed with this application.
4. If Buildings 25, 26, and 27 is allowed in the current proposed location, the travelway southwest of
building 27 must be redirected slightly and extended to the property boundary of TMP 32 -22P
[18-32.7.2.51
(Rev. 1) This comment has been withdrawn.
5. Please provide the date of the topographic information.
(Rev. 1) Comment has been addressed.
6. Please provide a benchmark on the plan.
(Rev. 1) Comment has been addressed.
7. Please show the stream buffers on all applicable sheets. Any stream buffer disturbance except
exempted items must be mitigated.
(Rev. 1) Comment has been addressed. However, additional disturbances in this plan must be
accounted for in the previously submitted mitigation plan for this project, WPO- 2009 - 00061. The
mitigation plan must be approved prior to the final site plan being signed. Revisions to the ESC
and Mitigation plan of this application are needed.
(Rev. 2) Comment remains unchanged. [17 -3221
8. Please show the approximate locations of the existing and new flood elevation lines.
(Rev. 1) Comment has been addressed.
9. On sheet C -8, please shade all critical slopes and show the approximate limits of critical slope
disturbance authorized by the Board of Supervisors at the time the rezoning plan was approved.
(Rev. 1) Comment has been addressed.
10. Please callout the end treatment for each guardrail section.
(Rev. 1) Comment has been addressed.
11. Please separate the drainage and stormwater management easements.
(Rev. 1) Comment has not been addressed. Pipes (excluding the riser culvert from facility 10 and
any pipe to or from a Vortechs or Stormfclter vault) or ditches conveying VDOT water require
public easements. SWM facilities will need a separate easement and will be privately maintained,
with the possible exception being SWM facility #10.
(Rev. 2) All SWM easements must be private.
12. The widths of the public drainage easements from structures 34.3 to outfall and 11 to 10 are not
correct. [DM]
(Rev. 1) The easement widths for pipes 11 -10, 34.3 -34.2, and 34.2 -34.1 are acceptable. However,
the drainage easements downstream of these pipes are not separated from the SWM easements as
required in the previous comment.
(Rev. 2) Comment has been addressed.
13. Please show all public drainage easements on the landscape plan to confirm that all significant
trees are located outside of the easements.
(Rev. 1) Comment has been addressed.
14. Please provide vertical profiles of the sight distance lines looking north from the entrances south of
buildings 11 and 19. The vertical alignment of Northwest Passage seems to obstruct the sight
lines. When the road is redesigned, make sure the vertical alignment is corrected to provide
adequate sight distance.
(Rev. 1) Comment has been addressed.
15. Please label each entrance with a VDOT designation.
(Rev. 1) Comment has been addressed.
16. Entrances cannot have a slope greater than 4% for the first 40ft from the edge of the curbline.
Please revise the spot elevations at the Discovery Court entrance so that the cross slope is as close
to 4% as reasonably possible. [18- 4.12.17]
(Rev. 1) Comment has been addressed.
17. VDOT approval of all road plans must be received before the final site plan can be approved by
the County.
(Rev. 1) VDOT approval has not yet been received.
(Rev. 2) VDOT approval has not yet been received.
18. Curbing is required on the landscaped area between parking spaces. Please either revise the site
plan or request a waiver from the Zoning Administrator per 18- 4.12.2.c. [18- 4.12.15.g]
(Rev. 1) Comment has not been addressed. To save space, I recommend eliminating every other
grassed island and provide 4 parking spaces centered on the wall separating the units. This
would maintain the applicant's desire to have each pair of spaces correlate to a unit and provide
a greater width of each landscape island.
(Rev. 2) Comment has been addressed.
19. All parking areas and travelways adjacent to parking areas must be no steeper than 5% (this
includes all "driveway" spaces adjacent to buildings). This maximum appears to be violated
consistently throughout the site. Please either revise the site plan so that no slope is over 5% or
request a waiver from the Zoning Administrator per 18- 4.12.2.c. Engineering review does not
recommend approval of this waiver. [18- 4.12.15.c]
(Rev. 1) A waiver of this ordinance standard has been approved by the Chief of Current
Development.
20. A few spaces on the south end of building 6 are less than 18ft long. Please rotate the building so
the length meets the minimum requirement for a parking space. [18- 4.12.16.c]
(Rev. 1) Comment has been addressed.
21. Please remove the 2 southernmost parking spaces east of building 1. The spaces are too close to
the entrance and create an issue when a vehicle is entering the site at the time another is backing
from these spaces. [DM and 18- 32.7.2]
(Rev. 1) Comment has been addressed.
22. The slope of the travelway adjacent to inlet 29 directs concentrated water across the drive aisle.
Please regrade the travelway or move inlet 29 so that it is immediately downhill of the nearby
filterra. [18- 32.7.2]
(Rev. 1) Comment has not been addressed. The grading in this area is the same; only a note was
added to the plans. Please show how (with contour lines and spot elevations) the contractor is to
grade the pavement to direct the spread flow from this curbline into DI -29. Or, please shift the
filterra to the north and relocate Inlet 29 before the travelway.
(Rev. 2) Comment has been addressed.
23. It appears that an inlet was omitted from the parking area west of building 25.
(Rev. 1) The revised grading is acceptable. However, the lack of an inlet in the area makes the
downstream inlet, 28, more critical. Similar to Inlet 29, the proposed grading indicates that flow
from the curbline will bypass this inlet.
(Rev. 2) Comment has been addressed.
24. The pipe from inlet 35.1 to 35 is at an acute angle. Please revise the network so that the change of
flow direction is at least 90 degrees. [DM]
(Rev. 1) Comment has been addressed.
25. Pull the guardrail behind the fire hydrant north of building 12.
(Rev. 1) Comment has been addressed.
26. Remove the sediment trap grading north of the entrance in all sheets but the ESC plan, if
necessary.
(Rev. 1) Comment has been addressed.
27. Where is Detail C on Sheet 49 located in the plan? This detail should be removed from the plan if
it is not needed.
(Rev. 1) Comment has been addressed.
28. The standard pavement section is adequate for most of the parking areas and travelways outside of
the ROW. However, it appears as though a few of the entrances may have ADT's greater than the
approximately 260 trips the standard payment section can handle. Please provide a detail that
shows the projected ADT's of each travelway and entrance so that the pavement sections can be
verified to be satisfactory. [18- 4.12.15.a]
(Rev. 1) Comment has been addressed.
29. Please identify the layback angle of the retaining wall in the detail and consider in plan view the
lost horizontal space as the wall height increases.
(Rev. 1) Comment has been addressed.
30. A bus stop is required on site for the southbound lane of Lewis and Clark Drive. [Proffer 9.2.a]
(Rev. 1) I have been informed that these bus stops are not ADA compliant with regard to
possessing an appropriate landing area. An 8ft deep by 5ft wide paved landing area is needed
around the front of each bus stop. The graphic below is a detail provided by JAUNT which
operates buses with a handicap lift at the rear of the vehicle. Since it is anticipated that
Charlottesville Area Transit will be servicing this area eventually, please coordinate the design of
these stops with their office.
(Exhibit Removed)
(Rev. 2) Comment has been addressed.
31. In note #6 on the drainage profile sheets, please clarify that the 4ft drop includes water falling
from the inlet to the bottom of the manhole.
(Rev. 1) Comment has been addressed.
32. For all curb inlets that do not have overland flow to SWM facilities, the sizing criteria must use
6.5in /hr. [policy]
(Rev. 1) Comment has not been addressed. Structures 11.1 and IL 2 must have 100% efficiency
since the downstream structures are not routed to a SWM facility. It appears this can be achieved
with throat lengths of 12ft. The longitudinal slope of the roadway at these structures appears to
be around 7 %, not 5 %. Please check all other inlet calculations to make sure the correct
longitudinal and cross slopes are applied when the roadway was regraded (the calculation table
is dated in December).
(Rev. 2) Comment has been addressed.
33. Structures 2 and 3 should be located at the low point of the roadway.
(Rev. 1) County Engineering will defer to VDOT on the design of the intersection with Route 29.
(Rev. 2) It appears as though a new inlet (2.1) was added to the plan to address the runoff at the
low point. The county concerns have been addressed. VDOT will review the hydraulic
computations as well.
34. I recommend reevaluating the placement of handicap ramps in parking areas where significant
runoff will be traveling in the curbline across ramps.
35. In the Landscape Plan, please provide a low maintenance, non - grassed groundcover on all slopes
steeper than 3:1. [DM]
(Rev. 1) Please hatch or shade all areas on the landscape plans steeper than 3:1 and refer to Note
31 on Sheet C -46.
(Rev. 2) Comment has been addressed.
36. (Rev. 1) Please grade a 6ft wide grass path to the conservation area from the sidewalk south of
Building 2 so the county can establish a trail to the greenway when the land is dedicated. From
the sidewalk to the top of the hill, it looks as though a 14% grade can be achieved.
(Rev. 2) The revised location of the greenway access point is acceptable. However, please widen
the public easement to include both of the sidewalks. The public access easement onto Girard
Lane and Dillon Court no longer seem necessary now that the public can access the greenway
completely from Girard Terrace.
37. (Rev. 2) The site plan is no longer in conformity with the application plan. In the previous two
final site plan submittals, the approved preliminary plan, and the approved application plan,
there were two access points onto Lewis and Clark Drive for the western half of the
development. In the latest submittal, one of these entrances was removed. This change also
runs counter to a requirement of the county ordinance which requires 50 or more units to have
two connections to a public street. [18- 32.7.2.41 The site plan must be revised to provide two
safe entrances onto Lewis and Clark Drive for the western half of this development. [18- 8.5.5.21
38. (Rev. 2) In the previous submittal a concrete sidewalk was provided without curb in front of all
of the driveway units (buildings 6, 8, and 9). In the latest submittal (buildings 1, 2, 6, 8, and 9),
the concrete was removed and it seems as though the applicant's intention is to have this area
be standard asphalt pavement. The area for pedestrians needs to be demarcated in someway.
There are several options for the applicant to consider. reverting back to a concrete walk
between the driveways and travelway as proposed in the first revision, hatching or striping the
pedestrian areas as a crosswalk, or using some architectural feature like painted, stamped
asphalt (as seen at some of the crosswalks in the north downtown area of Charlottesville). [18-
32.7.2.8]
C. Road Plan review comments (SDP- 2010 - 00021)
1. Before the final site plan can be approved, all road improvements as outlined in Proffer 5.3.1.c
must be approved and bonded. The WPO plans associated with these road improvements must
also be approved and bonded prior to site plan approval.
The design of two of these road improvements (i and iii) has been included in a plan that was
previously submitted to the county (WPO- 2009 - 00067). The other three proffered improvements
(ii, iv, and v) have not been included in any plan received by the county.
All easements (drainage, SWM, ESC /construction, etc.) and ROW associated with the
construction of the offsite road improvements must be platted prior to road plan approval.
[Proffer 5.3.1.c]
(Rev. 1) Comment has not been addressed.
(Rev. 2) Comment remains unchanged. The applicant has submitted a plan which has been
reviewed once for items ii and iv. The site plan cannot be approved until all road plans are
approved and bonded per this proffer.
2. Prior to the approval of the plans for improvements at any US Route 29 intersection, Owner shall
provide VDOT traffic signal network timing plans that VDOT finds acceptably address the
impacts of the proposed traffic signals for peak traffic periods. Please provide proof of this
approval from VDOT. [Proffer 5.3.21
(Rev. 1) Comment has not been addressed.
(Rev. 2) Comment has not been addressed. When asked in an email this week whether Proffer
5.3.3 (which I incorrectly referenced during the first review of the plan) was addressed, VDOT's
response was the following:
The attached e-mail is the signal plan that VDOT provided the developer to make their
adjustments to. VDOT still needs the signal plan along with a timing plan for the fourth leg
and an intersection analysis to determine if the intersection lane capacities are adequate.
VDOT has not yet received this information.
Furthermore, VDOT's third review sent March 18th included the following comment:
The lane configuration will need to be verified as adequate prior to permitting of
construction based on the proposed traffic signal timing plan and storage queues. VDOT
has not yet received this information for review and cannot comment on the adequacy of
the storage at this time.
This comment indicates that Proffer 5.3.3 of ZMA- 2000 -00009 has not been met. It also
signifies that Condition I of SP- 2006 -00034 has not been met either. (The county will defer to
VDOT in regard to the lane configuration).
Currently, there is some question as to whether the primary road section for Northwest Passage
(Lewis and Clark East) is acceptable to VDOT. If a modification to the typical road section is
required by VDOT, a variation must be submitted to the Planning Department so that cross - section
NWP3 as shown on sheet D1 of the Rezoning Plan may be altered to meet VDOT standards.
[Proffer 5.1]
(Rev. 1) The variation to modify the roadway cross - section is currently under review by the
Planning Department. The plan view sheets do not match the cross section for most of Northwest
Passage. If Planning grants the variation, please modify the plan to match the approved cross
section.
(Rev. 2) It is my understanding that the dimensions of the street cross - sections shown in this site
plan are acceptable to all parties (Current Development Engineering, the Planning
Department, and VDOT).
4. The Right of Way must be placed lft outside of the sidewalk and not on its edge.
(Rev. 1) Comment has not been addressed.
(Rev. 2) It is my understanding that VDOT will accept the ROW being placed 0.5ft outside of
the sidewalk.
5. In the construction set for the extension of Northwest Passage to North Pointe Blvd., the applicant
must design and construct a public road to the TMP's 32 -22P and 32 -22G. [18- 32.7.2.5 and 14-
409]
(Rev. 1) As a result of the meeting held on June 2e, 2010, this connection does not need to be
made with this plan. But, an entrance must be constructed as the applicant has promised to
include in the road plan to follow.
6. Please remove the temporary turnaround from all sheets since the roadway will be constructed to
North Pointe Boulevard.
(Rev. 1) Comment has been withdrawn.
7. Condition 7 of SP- 2006 -00034 has not been met.
(Rev. 1) Comment has not been addressed. All trees must be 2.5" caliper.
(Rev. 2) The 2.5" caliber plantings should be extended to Sta. 14 +60 as stated in my May 10" `
2010 email to Doug March after a discussion was had regarding whether more trees could be
provided closer to the intersection. The following is taken from that email.
I spoke with Gerald about the condition [ #7]. You do not need to plant any closer to Route
29. However, conditions 7 and 8 should be extended to Sta. 14 +60 to meet the 400ft spirit of
the condition since it cannot be met literally.
This appears to have been carried out for condition 8, but not condition 7.
Inlet 2 and 3 should be located at the low point of the road and the low point should be moved
farther into the site.
(Rev. 1) County Engineering will defer to VDOT on the design of the intersection with Route 29.
(Rev. 2) It appears as though a new inlet (2.1) was added to the plan to address the runoff at the
low point. The county concerns have been addressed. VDOT will review the hydraulic
computations as well.
9. Immediately prior to site plan approval, the applicant must submit a road bond request form for
each road plan. With each bond request, the applicant must fill out a schedule of completion. All
bonds (roads, swm, and esc) must be posted prior to final site plan approval. [Proffer 5.3.1.c]
(Rev. 1) Comment has been noted by the applicant.
(Rev. 2) To request any bond, please complete a Bond Estimate Request Form and submit it to
the County Engineer after all plans are approved. A road bond will require a fee of $250. To
post a road bond, a Schedule of Completion must be approved by the County Engineer.
D. SWM review comments (WPO- 2010 - 00017)
1. Approval from Filterra for the current design has been received. If any change to the Filterra
watersheds or placement occurs, an updated letter will be required.
(Rev. 1) Comment has been acknowledged by the applicant.
(Rev. 2) Comment has been acknowledged by the applicant. A new letter is not needed for this
latest submittal because Filterras were only eliminated, not added or modified significantly.
2. An approval letter from the manufacturer of the Stormfilter system is required.
(Rev. 1) Approval from Contech for the current design of the Stormfilters has been received. If
any change to the Stormfilter watersheds or placement occurs, an updated letter will be required.
(Rev. 2) Please provide an updated approval letter from Contech because the watersheds and
arrangement of the facilities have been modified since the last approval letter.
3. Please provide a stormwater facility maintenance agreement and fee for each property a facility is
located on.
(Rev. 1) Comment has not been addressed.
(Rev. 2) Comment has been addressed.
4. I have reviewed the detention waiver request with the County Engineer. The County Engineer will
grant the waiver of 2 and 10 year detention for the northern drainage area on the condition that
detention is provided upstream of both Stormfilter systems so that the water quality volume is
guaranteed to be routed through the water quality units. This must be demonstrated through
routings of rational method storms of varying durations.
(Rev. 1) The county has reviewed the documents provided by the applicant with the resubmittal.
These documents include the site specific approval letter from Contech, a copy of the Stormfilter
section of the current edition of the VSMH, and the customary 2004 design method explanation
letter which mentions the 0.35 in/hr design storm metric. I could not find any document from
Virginia DCR that mentioned this 0.35 in/hr intensity as a standard and I have requested DCR's
official opinion on this sizing method. I am currently awaiting a reply and will forward it to the
applicant once it is received. Until then, this comment is still pending. In order for me to feel
comfortable with this sizing concept without DCR approval, I would need to see an analysis of the
rainfall intensities in Virginia showing that at least 90% of annual rainfall occurs with intensities
of 0.35 in/hr or less. This kind of information is what I was expecting with the resubmittal.
(Rev. 2) The applicant has provided detention upstream for the Contech facilities by moving
one unit downstream of the riser for SWM 10 and by the addition of SWM 11 in the northwest
corner of the site and moving another Contech structure downhill of this facility. In late
November of 2010, the applicant had requested engineering staff, through Mark Graham, to
determine whether SWM facility 11 could be allowed within the conservation area with utilities
by simply leaving the grading of sediment basin I in place. The response from county
engineering was provided by Glenn Brooks on December 8 'h in an email written to Mr. Herb
White of WW Associates. The email stated that "the proposal to keep the temporary sediment
facility as a permanent facility...is problematic." Glenn further went on to say that "the
permanent basin also raises issues with other parts of the [county] organization," namely
Zoning (in regard to the site plan's overall compliance with the ZMA) and the ARB with regard
to the entrance corridor review. I understand that the ARB has conditionally approved the
facility. However, I do not believe Zoning has made an official determination regarding this
issue and will be doing so in the next week or two. I have reviewed the new SWM facility 11,
and have provided comments immediately below, assuming that the Zoning Department will
determine that the site plan still conforms to the application plan. Comments from the
modification to SWM 10 will continue in comment D.13.
The spillway for SWM #11 (and sediment basin 1) cannot go through the previously
undisturbed conservation area. Please direct the spillway to the outlet of the Contech
facility so only one riprapped channel to the stream is needed. The previous submittal
showed the spillway for sediment basin 1 terminating at the conservation easement line.
[SP- 2002 -00072 Condition #4, 17 -318, 17 -319, 17 -3201
The spillway is used more frequently than the average stormwater facility because there
is only one 15 "pipe outlet. The rocklined weir should be eliminated and replaced with
a concrete weir because of the uncertainties with the construction and routing of
riprapped spillways. Also, the VSMH requires that the armoring of a
principlelemergency spillway should begin at the upstream face, not at the top
elevation. [VSMH 3.02, 17 -303]
• Similar to what was done for the channel sections A -A, B -B, and C -C to SWM 10,
please provide calculations for the downstream emergency spillway and riprapped
channel below SWM 11 to make sure it will be non - erosive for the 2-year storm and
capable of carrying the 10 year flow within its banks.
• Please provide a graded access path to the facility embankment from the roadway.
Where the access path is steeper than 10 0/c, it must be graveled or paved. [Policy]
5. Please show all roof drain collectors on the site plan. Roof drain collectors are required for any
portion of any building that does not have overland flow to a drainage inlet.
(Rev. 1) Comment has not been addressed. Roofdrains are necessary for Buildings 1, 2, 9, 14, 18,
21, and 24.
(Rev. 2) Comment has been addressed.
6. The ARB must approve all design aspects of Stormwater Facility 10.
(Rev. 1) Comment has been acknowledged by the applicant.
(Rev. 2) It is my understanding that the ARB has approved the design of SWM facility 10. If
any modification to the facility is made, a new review of this facility may be required.
7. Water quality requirements for the site will be met if the applicant can provide enough detention to
route the entire water quality volume through the stormfilter systems. This must be demonstrated
through routings of rational method storms of varying durations.
(Rev. 1) The county has reviewed the documents provided by the applicant with the resubmittal.
These documents include the site specific approval letter from Contech, a copy of the Stormfilter
section of the current edition of the VSMH, and the customary 2004 design method explanation
letter which mentions the 0.35 in/hr design storm metric. I could not find any document from
Virginia DCR that mentioned this 0.35 in/hr intensity as a standard and I have requested DCR's
official opinion on this sizing method. I am currently awaiting a reply and will forward it to the
applicant once it is received. Until then, this comment is still pending. In order for me to feel
comfortable with this sizing concept without DCR approval, I would need to see an analysis of the
rainfall intensities in Virginia showing that at least 90% of annual rainfall occurs with intensities
of 0.35 in/hr or less. This kind of information is what I was expecting with the resubmittal.
(Rev. 2) Please refer to Comment D.4.
8. For the CN calculation for the post - development drainage area for SWM 10, use higher CN values
than 85 and 90 for townhouses because the impervious percentage for this site is greater than the
65% found in the VSMH table.
(Rev. 1) Area I and 2 in the SWM 10 pre- development table appear to be in the incorrect rows.
(Rev. 2) Comment has been addressed.
9. Stormwater Facility 10 must also provide detention for as much of the extension of Northwest
Passage (and surrounding land that drains to Northwest Passage) that the stubout from 35.3 would
collect. Once the full road plans for Northwest Passage are prepared, the detention calculations
can be evaluated again for compliance.
(Rev. 1) The post- development drainage area limits maintain the pre- development watershed,
which is unrealistic. At a minimum, please update the map and calculations to include all of the
bus parking.
(Rev. 2) Comment has not been addressed. The previous comment was referring to the bus
parking at the school site, not the bus stops (which were already within the watershed of SWM
10). It looks as though the drainage area will increase by approximately 17,OOOsf (with
— 10,OOOsf of impervious area).
10. For all curb inlets that do not have overland flow to SWM facilities, the sizing criteria must use
6.5in /hr. [policy]
(Rev. 1) Comment has not been addressed. Structures 11.1 and 11.2 must have 100% efficiency
since the downstream structures are not routed to a SWM facility. It appears this can be achieved
with throat lengths of 12ft. The longitudinal slope of the roadway at these structures appears to
be around 7 %, not 5 %. Please check all other inlet calculations to make sure the correct
longitudinal and cross slopes are applied when the roadway was regraded (the calculation table
is dated in December).
(Rev. 2) Comment has been addressed.
11. The downstream slope of the embankment of SWM facility 10 must be 3:1. [VSMH MS 3.01 -13]
When this change is made please make sure the embankment width is compliant with Table 3.01 -1
and the work is shown outside of the conservation area and floodplain.
(Rev. 1) Comment has been addressed.
12. If SWM -10 is to remain a dry detention basin, please provide a low flow channel to the riser from
all inlet points.
(Rev. 1) Comment no longer applies.
13. Proffer 9.1 states that the applicant will grant all permanent and temporary easements for the use
of SWM facility 10 when the county's school lot is developed. Although the proffers and plan do
not explicitly require the applicant of this development (NW residential area) to design or build
this facility for the school's stormwater runoff, engineering review understands that the approval
of the variation to allow buildings 25, 26 and 27 in the current proposed location is contingent
upon the design of several aspects of the land surrounding this facility.
As proposed, the intent of the design of SWM facility 10 appears to provide detention of only the
impervious area from this development, the Northwest Residential Area (though, see comment 9).
It can safely be assumed that the facility would require a considerable expansion in order to satisfy
a portion of the school's water quality and quantity requirements. There are two ways the facility
could be increased to meet SWM requirements for the future development of the school. Without
an easement from the property owner of TMP 32 -22P however, the only option for expansion of
the facility would be to the east towards Northwest Passage, which would likely require steep
slopes and retaining walls. Because the design of the school site has not be initiated and the
ultimate size and shape of the facility is unknown, the applicant must show a temporary
construction easement bounded by the property line with TMP 32 -22P (south), the conservation
line (west and northwest), the road ROW (east), and a line running from east -to -west at structure
34.1 from the ROW to the conservation area (north). The stormwater quality system must be
shown outside of this temporary construction easement. The construction easement will need to be
platted prior to approval of the final site plan. The plat should indicate that all of the construction
easement is reserved for possible future SWM dedication upon demand of the county when the
school lot is developed.
(Rev. 1) The drainage area to this facility is below the typical acreage needed to maintain a
sustainable water quality wet pond. Please provide a water balance calculation to determine
whether the wet pond will be drawn down after a 30 -day summer drought by an acceptable
amount. Because this is a facility that will be visible from nearly every direction (and subjected to
ARB review) and is just 4ft deep, any dry weather drawdown will have significant impacts to the
BMP. Section 6.2 of the new proposed SWM pond standards (available online) explains this
calculation in detail. Once this calculation is performed, please send this to me.
If the drawdown is acceptable, please provide a calculation showing that the aquatic bench area
is at least 15% of the surface area of the pond. Please also provide a planting plan for the
aquatic bench.
If the drawdown is not acceptable, Facility 10 should be designed as a biofilter. If the extension
of Northwest Passage from Sta. 27 +80 to North Pointe Blvd. is directed through the Stormfilter
system (by connecting structure 46 to structure 35), the grading of the facility could likely remain
as proposed with a 6in PVC pipe at the base of the riser and a slight adjustment to the 2 -year
orifice. When the school is constructed, the 6" pipe at the base would be extended as an
underdrain and the county would fill in the pond with gravel and biofilter mix as necessary to
treat the impervious area it directs to this facility.
In either case, please also provide an anti - vortex device on the riser and update the routing
calculations if the anti - vortex device affects the stage- discharge relationship.
(Rev. 2) The design of SWM 10 has been modifed between the last submittal and now. The
following comments are provided for SWM facility 10 based on the latest plan modification:
a. There is a discrepancy with the elevation of the emergency spillway between the plan
view and the detail.
b. The calculations will need to be updated based on comment D.9.
c. The applicant has intended that this facility to be an extended detention facility. Please
provide the 30 -hour drawdown calculation showing that the facility meets this design
standard. [VSMH 3.07]
14. The SWM bond will be calculated at the time of WPO plan approval.
(Rev. 1) Comment remains unchanged. With the next submittal, please provide a cost estimate
(including labor) for the Contech facilities.
(Rev. 2) The estimate for the Contech facilities has been received. To request a SWM bond
estimate, please provide the county engineer with a completed Bond Estimate Request Form
once all plans have been approved.
15. (Rev. 2) The Stormwater narrative has been modified since the last submittal. Please remove
any reference to MS -19 with regard to detention for the SWM 11 watershed. Instead, please
reference the conditional detention waiver granted by county engineering after the first review
of the plan. The one percent rule does not exempt any development from detention.
E. Site ESC review comments (WPO- 2010 - 00017)
1. The current ESC plan shows disturbance to critical slopes that were shown as being preserved in
the approved rezoning plan. Because of this, a critical slope waiver must be approved by the
Planning Commission before the current plan can proceed or the ESC plan must be redesigned to
stay within the limits shown on the plan approved by the Board of Supervisors. Engineering
review recommends placing the sediment basin on the access easement to the church property
(after coming to a mutually acceptable agreement with the property owner) and phasing the
construction of buildings 1 and 2.
(Rev. 1) The Director of Community Development has made the determination that a critical
slope waiver for the additional disturbance is not needed.
2. A portion of Sediment Basin 1 is currently proposed in the conservation area. The program
authority will not allow this disturbance inside the conservation area. Engineering review
recommends placing the sediment basin on the access easement to the church property (after
coming to a mutually acceptable agreement with the property owner) and phasing the construction
of buildings 1 and 2.
(Rev. 1) The Director of Community Development will allow disturbance to the conservation
area as proposed by the applicant.
3. Please identify separate limits of construction for both this ESC plan and the Stream Crossing Plan
(WPO- 2009 - 00061). Please provide notes on Sheets 24 and 25 in the area of the stream crossing
that refers to the previously approved plan. Please also refer to the previously approved stream
crossing plan in the construction sequence and eliminate the reference to guardrail demolition to
establish the entrance; this work will be covered by the other plan. This comment assumes that the
plans will not be combined.
(Rev. 1) Comment has been addressed.
4. A grading permit for this project will not be issued until the stream crossing is in place unless the
stream cross plan is combined with the site plan set.
(Rev. 1) This comment has been acknowledged by the applicant.
(Rev. 2) The technical aspects of the stream crossing ESC plan (WPO- 2009 - 00061) had been
approved in January of 2010 at the request of the applicant so an extension of the Army Corps
Permit could be received before an approaching deadline. That technical approval was not
given to all other aspects of the stream crossing plan: design of the culvert, FEMA approval,
mitigation approval, etc. All of these elements must be approved prior to the approval of this
site plan. [SP- 2006 -00034 Condition 2, SP- 2006 -00034 Condition 3, SP- 2006 -00034 Condition
5,17-3221
Per 17- 204.G, the approval of ESC plan for the stream crossing is now void. In the meeting
held between county staff and the applicant on June 24'" 2010, it was discovered that I had
mistakenly allowed disturbance to the conservation area for the construction of a sediment trap
and it was agreed upon that sediment basin I would be constructed with the stream crossing
WPO plan, but no plan was ever submitted to correct the sequence. Please submit a new
application, fee, and a set of the revised stream crossing ESC plan showing the construction of
sediment basin 1.
5. Per Proffer 4.3.a., this development is required to provide extra erosion and sediment control on
site to the satisfaction of the Program Authority. The applicant has identified 6 items that were
provided in this plan that he stated were above and beyond standard erosion and sediment control
practices. Those 6 items are: 1) All 2:1 slopes to have EC -2 lining, or annual rye hydroseed with
tackifier. 2) Use of wire - supported fence on the east side of the site bordering NF Rivanna River.
3) For E &S Control Phase 1, the volume of SB -2 is greater than the required minimum by
approximately 60 %. 4) For E &S Control Phase 1, the volume of SB -1 will be greater than the
required minimum by more than 200 %. 5) Existing sediment traps, silt fence and other E &S
control features installed under WPO- 2009 -0061 will remain in place for as long as feasibly
possible. 6) Temporary Slope drains will be installed from the outlets of SB -1 and SB -2.
Though engineering review considers at least half of these items normal requirements, we will
consider this proffer satisfied if the applicant extends the temporary slope drains from Basins 1 and
2 to the stream. The applicant must also place a note on sheets 24 and 25 that no heavy equipment
must be used to install this slope drain and associated riprap. [SP Condition #4]
(Rev. 1) Comment has been addressed.
6. The concept for sediment basin 2 does not seem to work during the intermediate stages between
the two phases. An additional phase to the plan is necessary to clarify the construction sequence.
Please provide a phase between the two existing phases that shows what the site would look like
the day before Sediment Basin 2 must be removed. The proposed layout, grading, and stormsewer
plan appears to require the basin to be removed or affected too soon. Similarly, the changes
required to address comments 1 and 2 will likely have the same issue which could be addressed
with this intermediate phase sheet.
(Rev. 1) The area west of the curb along the stormsewer system 28 to 23 cannot be solely
protected by silt fence to state standards because of the grading required and the total width of
disturbance. Though, when the filling of SB2 takes place, the use of silt fence as the sole measure
is unavoidable. To minimize the amount of time relying on silt fence in this area, the grading
necessary to establish pad for buildings 14 and 24 and the slopes to the west must be performed
while SB2 and the fill diversions are in place. Once these slopes are stabilized and the disturbed
area is minimized, then SB2 may be removed. Please clearly explain this procedure in step 12 of
the construction sequence. Engineering review recommends identifying the area on site where the
SB2 fill soil is to be stockpiled prior to the filling of the basin.
(Rev. 2) Comment has been addressed.
7. In the Phase I plan, please show the retaining walls needed to install the fill diversions being
constructed. The work to install the walls will be performed below the at -grade diversion dikes.
(Rev. 1) Comment has been addressed.
8. Please move the construction entrance for this ESC plan to just east of the diversions so that the
entrance can drain to a sediment trapping measure. The construction entrance in phase 1 should
be placed on the existing haul road and in phase 2 the entrance should be placed on Northwest
Passage. This comment assumes that the plans will not be combined and the grading permit for
this plan will be given after CRS and grass has stabilized the majority of the stream crossing plan.
(Rev. 1) Comment has not been addressed. Construction entrances are required to drain to a
sediment trap or basin per VESCH. In the first phase of the stream crossing plan, WPO -2009-
00061, this is difficult to achieve and there are no practical alternatives. Once final grading is
achieved at the stream crossing, the area should be stabilized with the gravel base and kept clean
by moving the construction entrances east of the ROW diversion so the tire washoff area can be
directed to a sediment trapping measure, either SBI or SB2, via diversions (or inlets and pipes in
Phase 2).
(Rev. 2) Comment has been addressed.
9. In the set, please include the paved wash rack detail found in the county's design manual, available
online, and remove the standard detail from the VESCH.
(Rev. 1) Comment has been addressed.
10. Please provide more DC (both phases) and PS (in Phase 2) symbols throughout the plan.
(Rev. 1) Comment has been addressed.
11. Please show a location for a staging and parking area on the plan.
(Rev. 1) Comment has been addressed.
12. Please provide a location for the soil stockpile on the plan. The erosion and sediment narrative
refers to an offsite stockpile and waste area plan on another North Pointe parcel. The stockpile
must be shown within the limits of an erosion and sediment control plan.
(Rev. 1) Comment has not been addressed. A stockpile is needed on the plan. The applicant is
referring to a waste area on an offsite parcel. Presumably, the contractor will need a
considerable amount of space to stockpile topsoil and cut, even if it is to be taken elsewhere. If it
is anticipated that cut will be hauled offsite, please state in the Offsite Area of the ESC narrative
that excess material is to be taken to a site with a permitted ESC plan.
(Rev. 2) Comment has been addressed.
13. Please clarify what Phase 1 mass grading is, as referenced in note 7 of the construction sequence.
(Rev. 1) Comment has been addressed.
14. Please lightly shade or hatch all critical slopes on sheet 24.
(Rev. 1) Comment has been addressed.
15. Please label the proposed drainage areas to basins 1 and 3 on sheet 25. My calculations show the
drainage area to Basin 1 being 10.6 acres in Phase 2.
(Rev. 1) Comment has been addressed.
16. The phase 2 portion of the ESC plan appears to have omitted the grading required at Sta. 26 +00 of
Northwest Passage.
(Rev. 1) Comment has been addressed.
17. For sediment basin 1, please show the 401 contour in plan view. The actual width of this
embankment is only 6ft and must be widened to 8ft. [VESCH MS 3.14]
(Rev. 1) Comment has been addressed.
18. Please provide safety fences surrounding all sediment basins stating "danger,quicksand, do not
enter."
(Rev. 1) Comment has been addressed.
19. Please provide the hydraflow routings of the sediment basin to confirm that the 25 -year storm is at
an acceptable elevation.
(Rev. 1) The 25 -year routing for Sediment Basin 3 does not appear to match the detail on Sheet C-
29. In the program, the crest is entered at 415.80 rather than the 416.50 exhibited on the detail.
Additionally, there is a discrepancy between the program and the detail with regard to the
contour areas of the facility.
For Sediment Basins I and 2, please confirm that the stage- discharge graph from Plate 3.14 -8
was used for the routing of the crest of each of the risers. The input for these two facilities
appears to assume that the top is a regular weir.
The dewatering orifice for Sediment Basin 1 on the detail does not match the routing.
(Rev. 2) Comment has been addressed.
20. For the CN calculation in the sediment basin calculations, use values of 82 (B soils) and 87 (C
soils) for exposed earth. Please also consider the impervious area in Phase H in basins 1 and 3
when checking the 25 -year storm elevation.
(Rev. 1) The use of 0.6 is acceptable and all facilities appear to pass the 25 year storm with
satisfactory freeboards.
21. The ESC bond will be calculated at the time of WPO plan approval.
(Rev. 1) Comment remains unchanged.
(Rev. 2) To request a ESC bond estimate, please provide the county engineer with a completed
Bond Estimate Request Form once all plans have been approved.
22. (Rev. 1) When additional dry storage is provided in a sediment basin the dewatering orifice
should be calculated from the height to the required volume, not provided volume. For instance,
in sediment basin 2 the 412.7cy is likely provided around the 402.8ft elevation. In this case, the
h/2 value would be 0.95ft rather than 1.3ft. For sediment basin 2, this might not be consequential
because a 4.0" orifice was provided anyway. Please update the calculations to confirm that a
4.0" inch orifice is appropriate. A
(Rev. 2) All dewatering orifice calculations are acceptable.
23. (Rev. 1) The dewatering orifice calculation for Sediment Basin 3 appears to be incorrect. The
contour areas do not match the values I received from the planimeter. The h/2 value should be
1.8ft (or, based on comment #22 above, around 1.5ft), rather than 0.6ft. The detail shows a 16"
reducer to be placed over the permanent SWM orifice, but this is not coordinated with the final
SWM plan. The only orifice proposed (in the detail on C -38 and in the routing output on C -34)
shows a 10.5 orifice with an invert of 410.25, while the dewatering orifice is set at 412.9.
(Rev. 2) Comment has been addressed.
24. (Rev. 1) Please specify an anti - vortex device on Sediment Basin 3 since the riser will be RCP.
What will the hydraulic effects of the anti - vortex device be for Sediment Basin 3 and SWM facility
10?
(Rev. 2) Comment has been addressed. The applicant has provided what he certifies to be an
antivortex device on the detail on Sheet 38.
25. (Rev. 2) The location of the emergency spillway for sediment basin I must match the location of
the permanent spillway for SWM 11. The spillway should be relocated to the south end of the
basin so that less disturbance to the conservation area occurs. [SP- 2002 -00072 Condition #4,
17 -318, 17 -319, 17 -3201